Cook noodles as package directs.
Meanwhile, melt butter in a large saucepan over medium heat. Add onion and cook until soft, 8 minutes. Add garlic and cook another 2 minutes. Reduce heat to a simmer, then add mustard and beer.
Drain noodles and add to the pan; stir to coat. Add 1/4 cup blue cheese, stirring gently to combine, and season with salt and pepper. Sprinkle remaining 1/4 cup blue cheese and the parsley on top.